Infrared study of CO adsorbed on Pt Al₂O₃. A method for determining metal-adsorbate interactions

When CO is adsorbed on platinum, two types of bonding are involved: a σ bond and a π bond obtained by back-bonding of metal "d" electrons to an antibonding orbital of CO. This model takes into account most of the experimental results obtained when CO is adsorbed on platinum. In the present paper, various gases having electron donor or electron acceptor properties have been adsorbed on platinum previously covered (θ = 0.2) by CO. When Lewis bases are adsorbed on platinum the effect of the adsorbed base is to increase the electronic density of the platinum crystallite and the back donation to the carbonyl group. This results in a shift of the ν(CO) frequency towards low wave numbers. This kind of interaction involves the collective properties of the metal. When atoms having electron acceptor properties, such as chlorine or oxygen, are fixed to platinum, the back donation is lowered resulting in a shift of the ν(CO) frequency towards high wave numbers. This kind of interaction is noticeable only when CO and the electron acceptor atom are fixed to the same platinum atom. In contrast with Lewis bases, the collective properties of the crystallites are not detected. The highest shift (Δν = 75 cm^-1) is observed when two chlorine atoms are adsorbed on the same platinum atom as CO. When benzene is adsorbed on platinum, the shift observed indicates a net "flow of charge" towards the platinum crystallites; this is a proof of the existence of a π complex between benzene and platinum as an intermediate state in benzene hydrogenation. The ν(CO) frequency shift technique can be applied to the study of various catalytic reactions on metals.